Dimethyl ether (DME) can be produced by one-step process and two-step process. The one-step process refers to one-step synthesis of dimethyl ether from the synthetic gas, and the two-step process refers to synthesis of methanol from the synthetic gas, and then preparation of dimethyl ether via dehydration.
The two-step process is carried out via two steps, i.e. synthesizing methanol from the synthetic gas, and then dehydrating methanol with the catalysis of an acid to prepare dimethyl ether. The two-step process for synthesis of dimethyl ether is the primary process for producing dimethyl ether at home and abroad. Said two-step process uses fine methanol as the feedstock, and has the advantages of less by-products of the dehydration reaction, high purity of dimethyl ether, mature technique, wide adaptability of the device, and simple post-treatment. Said two-step process can be directly used in a methanol factory, or other non-methanol factory having established public utilities. Generally, ZSM-5 molecular sieve comprising γAl2O3/SiO2 is used at home or abroad as the dehydration catalyst, wherein the reaction temperature is controlled at 280-340° C. and the pressure at 0.5 to 0.8 MPa. The single-pass conversion of methanol is from 70 to 85%; and the selectivity of dimethyl ether is greater than 98%.
CN1180064A discloses a process for producing dimethyl ether from methanol at a fairly low temperature (from 100 to 125° C.) and almost atmospheric pressure (from 0 to 0.05 MPa gauge pressure) in the presence of a new catalyst to produce a dimethyl ether gas.
CN1125216A discloses a process for producing dimethyl ether from methanol, comprising feeding methanol into the vaporization column to remove substances having a high boiling point and impurities, catalytically dehydrating in the presence of a composite solid acid catalyst in a multi-stage cold quenching reactor, then feeding the dehydrated product into a high performance package column for fractionation, and selecting different operation reflux ratios according to different requirements to produce a dimethyl ether product having a purity from 90 to 99.99%.
CN1368493A discloses a process for producing dimethyl ether by catalytically dehydrating methanol, and relates to a process for producing dimethyl ether by catalytic dehydration of methanol, wherein the dehydration is carried out in the presence of a solid acid catalyst containing SO42−. In the catalyst, SO42− is preferably in an amount from 2 to 25 wt %. The preferred catalyst support is selected from the group consisting of γ—Al2O3, η—Al2O3 and SiO2.
CN1301686A discloses a process for producing dimethyl ether by dehydrating methanol, comprising using sulfuric acid modified kaolin as a catalyst for the preparation of dimethyl ether via methanol dehydration.
US2004/0034255A1 discloses a process for producing dimethyl ether, which includes dehydrating methanol in vapor phase in the presence of an activated alumina catalyst having an average pore radius of 2.5 nm to 8.0 nm and having a sodium oxide content less than 0.07 wt %.
Said processes above primarily concern producing dimethyl ether by dehydrating methanol via catalysis with a composite solid acid, an acid-modified kaolin, an activated alumina, and the like. Moreover, a fixed bed reactor is mainly used therein. The resultant dimethyl ether is usually used as fine chemicals. In addition, said processes have a small scale of production and a higher production cost.
On the other side, the attempt of multipoint feeding has been carried out in various fixed bed methods or catalytic cracking methods. For example, U.S. Pat. No. 4,761,513 discloses a toluene alkylation method, comprising feeding the alkylation reagents from different sites of the fixed bed. In these methods, fairly big catalyst bed are needed to receive the reaction heat in order to prevent the potentially adverse effect of exothermic reaction on the product selectivity, resulting in a great increase of the device investment and operating cost. In addition, the reaction of producing dimethyl ether by dehydration of methanol is an exothermic reaction. Under the nearly adiabatic circumstance, the temperature of the catalyst bed layer gradually increases along with the proceedings of the reaction. If the reaction heat fails to be taken out or consumed timely, the pyrolytic reaction of methanol is prone to take place to produce much non-condensable gases, e.g. carbon oxides and hydrogen gas. Meanwhile, excessive high reaction temperature also results in the further dehydration of the resultant dimethyl ether to produce many low-carbon olefins, e.g. ethylene, propylene and butylene, so as to render notable decrease of the selectivity of dimethyl ether.